payment provider api

Greetings,
I saw that this issue has been only tackled in the past in this forum, and I would appreciate if someone give me reference about following issue.
As I figure out each payment provider (PayPal, Visa, etc...) has already their own established API-s, for on-line internet payment solutions, right???
Also I find that using DoDirectPayment API , extension of PayPal which implements in it meshanism for charging amount of various payment providers. It provide certain kind of solution for embedding such a peculiarity of web application (to utilize customer's charging)?
Am I correct, and does anyone could clarify these yet blur stuff for me???
thanks....